Princess Mako, 25, will lose her title and leave the cloistered imperial household to live with her husband in the outside world.
She will receive a one-off payment, after which the couple will be expected to provide for themselves. She will vote and pay tax, shop and do her own chores. If the couple have children, they will not be royal.
But her departure means one fewer to carry out official duties. It is also reigniting debate about the shrinking monarchy, the role women play in it and future succession.